Stack design



ATTORNEY INVENTOR. d B. TTG I" I W B TRAVER STACK DESIGN Flled Dec 281949 July 29, 1952 Patented July 29, 1952 Ward B. Traver", ijHammond,Ind, assignorito .Standard Oil Company, Chicago, 111., a :corpo j rationof Indiana This invention relates to an improved stack design and itpertains moreparticularlyto an improvement in the design of tall,cylindrical stacks, such as steel stacks whichfin meat senceof saidimprovement, would have a tend encytoward lateral vibration at aparticular-wind velocity. I

Application Dec ember 2 An object of theinvention is to provideacylindricaljstack which will avoid the-necessity of using uy wires orexpensive stiffening structurehwhen said stack is of such height'anddiameter that its natural frequency of lateral vi- ,bration is in therange of about 1 to 3 cycles per second. A further object is toprovideamethod andgmeans for stabilizing existing stacks against lateralvibration. Qther objects wi1l;be apparent as the detailed description ofthe invention proceedshb V .7 Y .Stackswhich are of conical section fromtop .to bottom presenttno'lateral vibration problem and this inventionis directed to cylindrical stacks, and to stacks having a tall,cylindrical portion"moun.ted on a conical base. The invention isparticularly applicable to stacks of about 2 to 15 feet or more indiameter and'with a cylindrical section. height of the order, of 50 to200 feet or more; Each such stackhas its particular natural frequency oflateral'vibration which can readily be calculated'by; those skilled inthe art. "When such natural frequency of vibration-is in the range ofabout 1 to ,3 cycles per second, particularly in the case of stackswhich are upwards of about 100 feet in height, it has long been knownthat there is a tendencyfor the stack to vibrate laterally at'adeterminable wind velocity. Heretofore it ,has been the practice toemploy guy wires to prevent such lateral vibration or to providestiffening structure required for altering the natural frequency of thestack.

.. I have discovered that such stacks can be protarded layer-of themoving air stream near the stack surface, and the geometric arrangementmust be such'as to break up this surface 'air stream: layer and createturbulence in order to avoid or minimize regular vortex formation. For

s, 1949; Serial No; 135,481

' '3 Claims. (01. 98-60) most stacks the protuberances should be fromabout one-fourth inch to' one' inch or more in height and theyare-preferably in the form of staggered short {strips of light metalspaced circumferentiallyat about- 10 to 15'. degree intervals or atdistances from one-half foot to two. feet apart. Close spacing of the,protuberances is more effective than the use of wider spacing but higherprotuberances. The protuberances may be formed of vertical-angle barsof: light metal and such angle bars may provide increased stiffening and're-enforcement, but for mypurpose such structural strength isimmaterial and staggered protuberances are even more effective thanangle bars. The protuberances maybe in the form'of woven'wirerorexpanded, metal around the upper portion of the stack providedcthat suchmaterial is of sufficientithickness to obtain" the required disturbancesof air flow vat the surface of the stack. w

The invention will be more clearlyiunderstoo d from thefollowingfdescr'iption of azspecific exstack showing protuberances'of'circular cross sectionarea; and. a l .7

Figure 5 is a detail;illustratingt thepuse of a fabric mesh protuberancearrangement. i ,3 g

In this particular example; thestack consists of a cylindrical portionllLrwhich-is 4 feet 3 inches in diameter and feetflhigh mounted on afrustro-conical steel base. I l,. 30 feethigh and 7 feet in diameter atits base; In this case the stackwall thicknesslis about e inch. The,stack base in this case is bolted to concrete founda- "tion and thenatural frequency of la'teralivibration for this particular stack isshown bytcalculation to be about 1.6 cyclesper' second. :7 e a If such astack were notre-enforced by" guy wires or stifiened by expensiveare-enforcing means, it would be subject to suchintensive lateralvibration at a windvelocity of about 24 miles per hour as to be indanger ofrupture or collapse. At low wind velocities of the order of onemile per hour, the vortices 'on each side down-stream would berelatively equal and of no serious effect; At higherwind velocitiesthere protuberances.

where F is the vortex frequency in cycles per second, V is the windvelocity in miles per hour and D is the stack diameter in feet.- y'I'hisalter: H

nate vortex formation has no serious effect, toward causing stackvibration until said frequency substantially equals the naturalfrequency of the stack in lateral vibration. In this particular example,when the wind velocity reaches 24.. miles per hour, the frequency ofvortex formation will be in resonance-with the stacksnatural frequencyof lateral vibration andathe net result will be a gradual increase inamplitude of vibration. which may become sosevere as to cause failureofthe stack. If the wind velocity exceeds 24 miles per hour, thefrequency of vortex forma- 25 tion will nolonger be in resonance withthe stacks natural frequency and at wind velocities sufficiently high toresult in turbulent flow, there is no longer a vibration problem.

Referring again to the drawing, it will be noted that the upper half ofthe stack is provided with protuberances l2 which in this particularexample are thin metal strips about 6 inches longand inch wide. Thesestrips are spot welded, brazed or otherwise secured to the stack so thatthey are each in a vertical position but are spaced from each other byabout. one foot andare staggered or geometrically arranged so as to givemaximum disturbance to the surface layer of air which is flowing pastthe stack. With such protuberances, which in this case are firinch high,relatively closely spaced and geometrically arranged to disturb surfaceflow, the tendency toward vortex formation is markedly altered. I Withsuch protuberances on the upper part of the stack, turbulent flowconditionsare obtained with a wind velocity so low that the frequency ofvortex formation cannot attain-theflstacks natural frequency ofvibration so that there is no longer any tendency for the stack tovibrate at the wind velocity of 24 miles per hour or in fact at anyother wind velocity. In other words, by roughening the surface of atleast the upper part of the stack in the defined manner, I lower I themaximum possible frequency of vortex formation to such extent that it isless than the stacks natural frequency of lateral vibration,

I have solved the vibration problem by'alterin fluid flow past the stacksurface instead of by altering the stiffness of the stack itself orsecuring the stack against vibration by means-0f guy wires. 1 H

In the above specific example, I employ geometrically spaced metalstrips. for distributing the surface flow ofv air past the stack but'itshould 85 be understood that the disturbance of surface flow may beeffected by various other types of Such protuberances may be along theentire cylindrical portion of the stack but usually it is only necessarythat the upper half orupper one-third of the stack be provided withprotuberances because of the higher wind velocities and the greatereffectiveness of the force exerted on the, stacks upper end. The 1vertical strips may extend from top to bottom in Fig.4. Theprotuberances may be in the form of a woven wire fencing or expandedmetal lath fabric I6 as illustrated in Fig. '5. Such woven wire fencingor expanded metal lath is not as effective as the strip protuberanceswhich extend at least about one-quarter inch to an inch or more from thestack surface unless the expanded metal iior woven wire extendsoutwardly far enouglitodisrupt the retarded surface-layer of the airstream flowing around the stack (in most 5 cases at "least aboutone-quarter inch).

The reason for having the protuberances at spaced intervals around thecircumference of the stack is to protect-the stack against lateralvibration regardless of wind direction. If a stack were in a locationwhere the wind was always from the same direction then it would only benecessary to have protuberances on each side of the stack about 70 tofrom the direction of the wind on the up-stream side. If strips or vaneswere movably mounted and directed, for exampleby a wind vane so that thestrips would always'be at about 70 to 80 from the direction of the windon the up-stream side, the objects of my invention could be accomplishedwithout employing the strips at spaced intervals around the entirestack. For practical purposes, however, it is usually desirable toemploy fixed protuberances at suflzlciently close intervals so that theretarded surface or boundary layer flow isnecessarily disruptedregardless of wind direction. The boundary layer thickness of a flowingair stream is somewhat dependent on wind velocity, but at any particularvelocity it increases in depth with increase in stack diameter. Theminimum protuberance depth for a 2 foot diameter stack is approximatelyone-quarter inch, but protuberances of greater depth maybe employed. Theminimum protuberance depth for a. ten foot diameter stack is aboutthree-quarter inch... In all cases the protuberances may be two or.three inches, but the use-of higher protuberances may be objectionablebecause of increased drag;

I claim? 1 1. A :cylindrical stack. having a natural frequency'oflateral vibration in the range of about 1 to 3 cycles per second andhaving protuberances on at least the upper portion of its externalsurface, sa-id protuberances being at least about onequar'ter inch toone inch in depth and being spaced laterally around the stack atintervals of about six inches to twofeet whereby said protuberancesdisrupt the surface air stream flowing past the stack and interfere withregularly alternating formations of vortices on opposite sides of thestack.

2. The cylindrical stack of claim 1 wherein the protuberances consist ofvertically aligned flat strips attached-tangentially to the surface ofthe stack of 'suiflcient width so that their free edges are-about oneinch from the surface of the stack, spaced laterally around the stack atintervals of about ten to forty-five degrees.

3; The cylindrical stack of claim 1 wherein the protuberances consist ofa sleeve of wire fabric around'the stack wherein the thickness of thefabric is not less than about one-quarter inch.

' WARD B. TRAVER.

N 0' references cited.

